🔁 Bash: Sourcing Files – Use source and . to Load Scripts Without Starting a New Shell

🧲 Introduction to Sourcing in Bash – Reuse Scripts with source or .

Bash allows you to load and execute another script into the current shell using the source command or its shorthand . (dot). This process is called sourcing, and it’s useful when you want to reuse variables, functions, or environment settings defined in another file without launching a new shell process.

Unlike running a script with ./script.sh or bash script.sh, sourcing keeps everything in the same shell session.

🎯 In this article, you’ll learn:

• What source and . do in Bash
• The difference between sourcing and executing
• When and why to use sourcing
• Real-world examples using environment variables and function libraries

⚡ What Does source Do in Bash?

The source command reads and executes the contents of a file in the current shell, not in a subshell.

📜 Syntax:

source filename

or simply:

. filename

🧠 Both forms are identical—source is more readable; . is more compact.

🧪 Example 1: Source a File with Variable Definitions

# config.sh
NAME="Vaibhav"
ENV="production"

# main.sh
source config.sh
echo "Running as $NAME in $ENV mode"

✅ Output:

Running as Vaibhav in production mode

🔁 These variables are now available in main.sh without redefinition.

🧪 Example 2: Source a File with Functions

# utils.sh
log() {
  echo "[INFO] $1"
}

# main.sh
source utils.sh
log "Script started"

✅ Output:

[INFO] Script started

📦 Sourcing is great for modular script design—split functions into reusable files.

🧪 Example 3: Use . Instead of source

. ./config.sh

✅ Same effect as source config.sh.

🔍 Sourcing vs Executing a Script

⚠️ When to Use Sourcing (and When Not To)

✅ Use Sourcing:

• To import functions, aliases, or env variables
• In interactive sessions to apply profile changes (e.g., .bashrc)
• When sharing logic across multiple scripts

❌ Avoid Sourcing:

• If the file modifies shell behavior destructively
• If isolation is needed (e.g., you don’t want its variables/functions to persist)

🧠 Best Practices for Sourcing

🧪 Example: Check Before Sourcing

CONFIG_FILE="./config.sh"
if [[ -f "$CONFIG_FILE" ]]; then
  source "$CONFIG_FILE"
else
  echo "Missing config file!"
  exit 1
fi

📌 Summary – Bash source and . Usage

The source and . commands let you reuse shell code—variables, functions, configurations—without running a new process. This enables modular, DRY Bash scripting practices across scripts and environments.

🔍 Key Takeaways:

• source or . executes code in the current shell session
• Great for importing variables, functions, and settings
• Does not start a subshell like bash script.sh
• Helps split scripts into maintainable, reusable modules

⚙️ Real-world Uses:

• Loading config files in deployment scripts
• Sharing utility functions across scripts
• Setting up interactive shell environments

❓ FAQ – Bash source and . Usage

❓ Is source the same as . in Bash?
✅ Yes. source filename and . filename are functionally equivalent in Bash.

❓ Can I source a script from another directory?
✅ Yes:

source /path/to/script.sh

❓ What happens if I run a script with bash script.sh instead of source?
✅ A new shell runs the script, and changes (like variables) won’t affect the current shell.

❓ How do I check if a file has already been sourced?
✅ Use a marker variable:

[[ -z "$CONFIG_LOADED" ]] && source config.sh && CONFIG_LOADED=1

❓ Can I use source in a cron job?
✅ Not directly recommended—cron runs non-interactively. Instead, set variables inside the script, not via sourced files unless full paths and environments are guaranteed.